Splash is a MVC PHP framework. It is making a heavy use of annotations, and of the Mouf dependency injection framework. You might want to use Splash in order to seperate cleanly the controllers (that perform the actions required when you navigate your web application) and the view (that generates and displays the HTML that makes your web pages).
This package contains the common functions used by all libraries respecting the Splash syntax (annotations, etc...) All libraries you said? Yes, all libraries. Indeed, there are many packages respecting the Splash syntax.
Of course, there is Splash (a full-featured MVC framework). But there is also Druplash (a module for Drupal that adds a Splash-compatible MVC framework),. and Moufpress (a plugin for Wordpress that adds a Splash-compatible MVC framework).
To learn more about Splash, check the documentation on the Splash website